U.S. patent application number 11/839421 was filed with the patent office on 2008-02-28 for adaptive selection of transmission parameters for reference signals.
Invention is credited to Tarik Muharemovic, Aris Papasakellariou.
Application Number | 20080051125 11/839421 |
Document ID | / |
Family ID | 39197294 |
Filed Date | 2008-02-28 |
United States Patent
Application |
20080051125 |
Kind Code |
A1 |
Muharemovic; Tarik ; et
al. |
February 28, 2008 |
Adaptive Selection of Transmission Parameters for Reference
Signals
Abstract
An embodiment of the present invention uses estimates of delay
spreads of transmissions from user equipments (UEs) to a NodeB to
determine a set of transmission parameters for the UEs reference
signals. In an exemplary embodiment, the transmission parameters
for reference signals include cyclic shifts. Thus, embodiments
include a set of allocated cyclic shift values that are tailored to
the delay spreads. The set of allocated cyclic shift values are
used by a corresponding set of UE being served by a NodeB to form
references signals. Each UE uses the allocated cyclic shift to form
its reference signal by applying the cyclic shift to a modified
reference sequence. The modified reference sequence can be
generated from a Constant-Amplitude-Zero-Auto Correlation (CAZAC)
sequence. The set of allocated cyclic shift values can be updated
periodically to account for changes of delay spreads, which can be
caused by physical movements of the set of UEs.
Inventors: |
Muharemovic; Tarik; (Dallas,
TX) ; Papasakellariou; Aris; (Dallas, TX) |
Correspondence
Address: |
TEXAS INSTRUMENTS INCORPORATED
P O BOX 655474, M/S 3999
DALLAS
TX
75265
US
|
Family ID: |
39197294 |
Appl. No.: |
11/839421 |
Filed: |
August 15, 2007 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60823211 |
Aug 22, 2006 |
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Current U.S.
Class: |
455/519 |
Current CPC
Class: |
H04W 72/1268 20130101;
H04W 72/1231 20130101 |
Class at
Publication: |
455/519 |
International
Class: |
H04Q 7/20 20060101
H04Q007/20 |
Claims
1. A method for defining transmission parameters of user equipment
(UE) reference signals (RS), comprising estimating channel delay
spreads of a plurality of UEs scheduled for transmission at a
particular transmission time period or sub-frame in an uplink for
communication with a NodeB; and allocating transmission parameters
to each scheduled UE of the plurality of UEs in accordance to the
delay spreads of the plurality of UEs scheduled for transmission in
the particular time period or sub-frame
2. The Method of claim 1 wherein the parameters are cyclic
shifts.
3. The method of claim 1, further comprising using the RS for
channel and CQI estimation.
4. The method of claim 3, wherein estimating delay spreads further
comprises periodically receiving a baseline reference signal from
each of the plurality of UEs at the NodeB for use in estimating the
delay spread of the corresponding UE, and wherein the baseline
reference signal is different than the RS.
5. The method of claim 1 further comprising sending cyclic shift
allocations to the plurality of scheduled UEs from the NodeB using
downlink control signaling.
6. The method of claim 2 wherein the NodeB allocates cyclic shifts
to the plurality of scheduled UEs, the allocating comprising:
selecting a first UE of the plurality of UEs and allocating to it a
cyclic shift of zero; and individually selecting each of the
remaining UEs of the plurality of UEs and allocating a cyclic shift
to an m-th UE based on a sum of cyclic shifts allocated to previous
m-1 UEs of the plurality of UEs scheduled for uplink transmission
during the same transmission time interval or sub-frame, where
m=[1, total number of UEs scheduled for uplink transmission].
7. The method of claim 6, wherein the cyclic shift allocated to
each m-th UE is equal to the sum of a delay spread and a timing
uncertainty of each of the previous m-1 UEs.
8. The method of claim 2, further comprising determining a base
cyclic shift value to provide orthogonal RS multiplexing relative
to the UE with the largest delay spread; and wherein the cyclic
shift allocated to each m-th UE is an integer multiple of the base
cyclic value.
9. The method of claim 8, wherein different base cyclic values can
be selected in different operating communication environments.
10. A method to adaptively select a cyclic shift applied to a CAZAC
sequence for the formation of an orthogonal reference signal (RS)
transmitted by a user equipment (UE), comprising: estimating delay
spreads of a plurality of UEs scheduled for transmission at a
particular transmission time period or sub-frame in an uplink for
communication with a NodeB; allocating a cyclic shift to each
scheduled UE of the plurality of UEs in accordance to the delay
spreads of the plurality of UEs scheduled for transmission in the
particular time period or sub-frame, such that two or more of the
UEs are allocated different non-zero cyclic shift values; and
receiving a plurality of reference signals from the respective
plurality of UEs, wherein each RS comprises a CAZAC sequence that
is cyclically shifted by an amount allocated to each respective UE
of the plurality of UEs, whereby each received RS is orthogonal to
each RS transmitted by all others of the plurality of UEs scheduled
in the same time transmission period or sub-frame.
11. The method of claim 10, wherein estimating delay spreads
further comprises periodically receiving a baseline reference
signal from each of the plurality of UEs at the NodeB for use in
estimating the delay spread of the corresponding UE.
12. The method of claim 10, further comprising using each RS for
channel and CQI estimation.
13. The method of claim 11, wherein the baseline reference signal
is different than the RS.
14. The method of claim 10 further comprising sending cyclic shift
allocations to the plurality of scheduled UEs from the NodeB using
downlink control signaling.
15. The method of claim 10 where the NodeB allocates cyclic shifts
to the plurality of scheduled UEs, the allocating comprising:
selecting a first UE of the plurality of UEs and allocating to it a
first cyclic shift value; and individually selecting each of the
remaining UEs of the plurality of UEs and allocating a cyclic shift
to an m-th UE based on a sum of cyclic shifts allocated to previous
m-1 UEs of the plurality of UEs scheduled for uplink transmission
during the same transmission time interval or sub-frame, where
m=[1, total number of UEs scheduled for uplink transmission].
16. The method of claim 15, wherein the cyclic shift allocated to
each m-th UE is equal to the sum of a delay spread and a timing
uncertainty of each of the previous m-1 U Es.
17. The method of claim 10, further comprising determining a base
cyclic shift value to provide orthogonal RS multiplexing relative
to the UE with the largest delay spread; and wherein the cyclic
shift allocated to each m-th UE is an integer multiple of the base
cyclic value.
18. The method of claim 17, wherein different base cyclic values
can be selected in different operating communication
environments.
19. A method to adaptively select a cyclic shift applied to a CAZAC
sequence for the formation of an orthogonal reference signal (RS)
transmitted by a user equipment (UE), comprising: transmitting a
baseline reference signal from a UE to a NodeB that is used to
estimate a delay spread during transmission of a sub-frame from the
UE; receiving at the UE a first allocated cyclic shift in
accordance to delay spreads of a plurality of UEs scheduled for
transmission in the particular time period or sub-frame; utilizing
at the UE the allocated cyclic shift to form the transmitted RS by
applying the cyclic shift to the CAZAC sequence; periodically
receiving at the UE an updated allocated cyclic shift that has a
different value in response to dynamic changes to delay spreads of
the plurality of UEs scheduled for transmission in a later
particular time period or sub-frame; and utilizing at the UE the
updated allocated cyclic shift to form a later transmitted RS by
applying the updated cyclic shift to the CAZAC sequence.
20. The method of claim 19, wherein the RS is used for channel and
CQI estimation.
21. The method of claim 19 wherein the allocated cyclic shift is
received from a serving NodeB using downlink control signaling.
22. A user equipment(UE) comprising: transmitter circuitry operable
to transmit a baseline reference signal from the UE to a NodeB that
is used by the NodeB to estimate a delay spread during transmission
of a sub-frame from the UE; receiving circuitry operable to receive
a first allocated cyclic shift in accordance to delay spreads of a
plurality of UEs scheduled for transmission in a particular time
period or sub-frame; processing circuitry connected to the
transmitter circuitry and to the receiver circuitry operable to
utilize the allocated cyclic shift to form a reference signal (RS)
for transmission by applying the cyclic shift to a CAZAC sequence;
wherein the receiving circuitry periodically receives an updated
allocated cyclic shift that has a different value in response to
dynamic changes to delay spreads of the plurality of UEs scheduled
for transmission in a later particular time period or sub-frame;
and wherein the processing circuitry is further operable to utilize
the updated allocated cyclic shift to form a later transmitted RS
by applying the updated cyclic shift to the CAZAC sequence.
23. A method to adaptively select a cyclic shift applied to a CAZAC
sequence for the formation of an orthogonal reference signal (RS)
transmitted by a user equipment (UE), comprising: estimating delay
spreads of a plurality of UEs scheduled for transmission at a
particular transmission time period or sub-frame in an uplink for
communication with a NodeB; allocating a cyclic shift to each
scheduled UE of the plurality of UEs in accordance to the delay
spreads of the plurality of UEs scheduled for transmission in the
particular time period or sub-frame, such that two or more of the
UEs are allocated different non-zero cyclic shift values; and
utilizing the allocated cyclic shift to form the transmitted RS by
applying a corresponding cyclic shift to the CAZAC sequence,
whereby the transmitted RS is orthogonal to each RS transmitted by
all others of the plurality of UEs scheduled in the same time
transmission period or sub-frame.
24. The method of claim 23, wherein estimating delay spreads
further comprises periodically transmitting a baseline reference
signal from each of the plurality of UEs to the NodeB for use in
estimating the delay spread of the corresponding UE.
Description
CLAIM OF PRIORITY
[0001] This application for Patent claims priority to U.S.
Provisional Application No. 60/823,211 entitled "Adaptive Cyclic
Shift Selection of Reference Signals in SC-FDMA Systems" filed Aug.
22, 2006, incorporated by reference herein.
FIELD OF THE INVENTION
[0002] Embodiments of this invention generally relate to wireless
communication and in particular to generation of reference signals
sent by mobile users.
BACKGROUND OF THE INVENTION
[0003] The Global System for Mobile Communications (GSM: originally
from Groupe Special Mobile) is currently the most popular standard
for mobile phones in the world and is referred to as a 2G (second
generation) system. Universal Mobile Telecommunications System
(UMTS) is one of the third-generation (3G) mobile phone
technologies. Currently, the most common form uses W-CDMA (Wideband
Code Division Multiple Access) as the underlying air interface.
W-CDMA is the higher speed transmission protocol designed as a
replacement for the aging 2G GSM networks deployed worldwide. More
technically, W-CDMA is a wideband spread-spectrum mobile air
interface that utilizes the direct sequence Code Division Multiple
Access signaling method (or CDMA) to achieve higher speeds and
support more users compared to the older TDMA (Time Division
Multiple Access) signaling method of GSM networks.
[0004] Orthogonal Frequency Division Multiple Access (OFDMA) is a
multi-user version of the popular Orthogonal Frequency-Division
Multiplexing (OFDM) digital modulation scheme. Multiple access is
achieved in OFDMA by assigning subsets of sub-carriers to
individual users. This allows simultaneous low data rate
transmission from several users. Based on feedback information
about the channel conditions, adaptive user-to-sub-carrier
assignment can be achieved. If the assignment is done sufficiently
fast, this further improves the OFDM robustness to fast fading and
narrow-band co-channel interference, and makes it possible to
achieve even better system spectral efficiency. Different number of
sub-carriers can be assigned to different users, in view to support
differentiated Quality of Service (QoS), i.e. to control the data
rate and error probability individually for each user. OFDMA is
used in the mobility mode of IEEE 802.16 WirelessMAN Air Interface
standard, commonly referred to as WiMAX. OFDMA is currently a
working assumption in 3GPP Long Term Evolution (LTE) downlink.
Also, OFDMA is the candidate access method for the IEEE 802.22
"Wireless Regional Area Networks".
[0005] NodeB is a term used in UMTS to denote the BTS (base
transceiver station). In contrast with GSM base stations, NodeB
uses WCDMA or OFDMA as air transport technology, depending on the
type of network. As in all cellular systems, such as UMTS and GSM,
NodeB contains radio frequency transmitter(s) and the receiver(s)
used to communicate directly with the mobiles, which move freely
around it. In this type of cellular networks the mobiles cannot
communicate directly with each other but have to communicate with
the BTSs
[0006] Traditionally, the NodeBs have minimum functionality, and
are controlled by an RNC (Radio Network Controller). However, this
is changing with the emergence of High Speed Downlink Packet Access
(HSDPA), where some logic (e.g. retransmission) is handled on the
NodeB for lower response times and in 3GPP LTE (a.k.a. E-UTRA)
almost all the RNC functionalities have moved to the NodeB.
[0007] The utilization of cellular technologies allows cells
belonging to the same or different NodeBs and even controlled by
different RNC to overlap and still use the same frequency. The
effect is utilized in soft handovers.
[0008] Since WCDMA and OFDMA often operates at higher frequencies
than GSM, the cell range is considerably smaller compared to GSM
cells, and, unlike in GSM, the cells' size is not constant (a
phenomenon known as "cell breathing"). This requires a larger
number of NodeBs and careful planning in 3G (UMTS) networks. Power
requirements on NodeBs and UE (user equipment) are much lower.
[0009] A NodeB can serve several cells, also called sectors,
depending on the configuration and type of antenna. Common
configuration include omni cell (360.degree.), 3 sectors
(3.times.120.degree.) or 6 sectors (3 sectors 120.degree. wide
overlapping with 3 sectors of different frequency).
[0010] High-Speed Packet Access (HSPA) is a collection of mobile
telephony protocols that extend and improve the performance of
existing UMTS protocols. Two standards HSDPA and HSUPA have been
established. High Speed Uplink Packet Access (HSUPA) is a
packet-based data service of Universal Mobile Telecommunication
Services (UMTS) with typical data transmission capacity of a few
megabits per second, thus enabling the use of symmetric high-speed
data services, such as video conferencing, between user equipment
and a network infrastructure.
[0011] An uplink data transfer mechanism in the HSUPA is provided
by physical HSUPA channels, such as an Enhanced Dedicated Physical
Data Channel (E-DPDCH), implemented on top of the uplink physical
data channels such as a Dedicated Physical Control Channel (DPCCH)
and a Dedicated Physical Data Channel (DPDCH), thus sharing radio
resources, such as power resources, with the uplink physical data
channels. The sharing of the radio resources results in
inflexibility in radio resource allocation to the physical HSUPA
channels and the physical data channels.
[0012] The signals from different users within the same cell may
interfere with one another. This type of interference is known as
the intra-cell interference. In addition, the base station also
receives the interference from the users transmitting in
neighboring cells. This is known as the inter-cell interference
[0013] When an orthogonal multiple access scheme such as
Single-Carrier Frequency Division Multiple Access (SC-FDMA)--which
includes interleaved and localized Frequency Division Multiple
Access (FDMA) or Orthogonal Frequency Division Multiple Access
(OFDMA)13 is used; intra-cell multi-user interference is not
present. This is the case for the next generation UMTS
enhanced-UTRA (E-UTRA) system13 which employs SC-FDMA13 as well as
IEEE 802.16e also known as Worldwide Interoperability for Microwave
Access (WiMAX)13 which employs OFDMA, In this case, the fluctuation
in the total interference only comes from inter-cell interference
and thermal noise which tends to be slower. While fast power
control can be utilized, it can be argued that its advantage is
minimal.
[0014] In the uplink (UL) of OFDMA frequency division multiple
access (both classic OFDMA and SC-FDMA) communication systems, it
is beneficial to provide orthogonal reference signals (RS), also
known as pilot signals, to enable accurate channel estimation and
channel quality indicator (CQI) estimation enabling UL channel
dependent scheduling, and to enable possible additional features
which require channel sounding.
[0015] Channel dependent scheduling is widely known to improve
throughput and spectral efficiency in a network by having the Node
B, also referred to as base station, assign an appropriate
modulation and coding scheme for communications from and to a user
equipment (UE), also referred to as mobile, depending on channel
conditions such as the received signal-to-interference and noise
ratio (SINR). In addition to channel dependent time domain
scheduling, channel dependent frequency domain scheduling has been
shown to provide substantial gains over purely distributed or
randomly localized (frequency hopped) scheduling in OFDMA-based
systems. To enable channel dependent scheduling, a corresponding
CQI measurement should be provided over the bandwidth of interest.
This CQI measurement may also be used for link adaptation,
interference co-ordination, handover, etc.
[0016] One method for forming reference signals is described in US
patent application 20070171995, filed Jul. 26, 2007 and entitled
"Method and Apparatus for Increasing the Number of Orthogonal
Signals Using Block Spreading" and is incorporated by reference
herein. The generation of reference signals (RS) sequences can be
based on the constant amplitude zero cyclic auto-correlation
(CAZAC) sequences, and the use of block spreading for multiplexing
RS from multiple UE transmitters is described therein.
SUMMARY OF THE INVENTION
[0017] Embodiments of the present invention use dynamically
estimated channel delay spreads of mobile users, to assign
parameters which define transmissions of reference signals (RS).
Exemplary embodiments determine a set of allocated cyclic shift
values that are tailored to the channel delay spreads. The set of
allocated cyclic shift values are used by a corresponding set of
user equipment (UE) being served by a NodeB to form reference
signals. Each UE uses the allocated cyclic shift to form its
reference signal by applying the cyclic shift to a reference
sequence. In some embodiments, the reference sequence is a
modulated Constant-Amplitude-Zero-Auto Correlation (CAZAC)
sequence. The set of allocated cyclic shift values can be updated
periodically to compensate for changes in UEs delay spreads.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] Particular embodiments in accordance with the invention will
now be described, by way of example only, and with reference to the
accompanying drawings:
[0019] FIG. 1 is a representation of two cells in a cellular
communication network that includes an embodiment of adaptive
cyclic shifting of references signals;
[0020] FIGS. 2A and 2B show two exemplary sub-frame structures that
include reference signals according to an embodiment of the present
invention;
[0021] FIG. 3 is a block diagram of an SC-FDMA system for
transmitting the sub-frame structures of FIG. 1;
[0022] FIGS. 4A and 4B illustrate alternative embodiments of
sub-carrier mapping in the system of FIG. 3;
[0023] FIG. 5 illustrates exemplary delay spread plots and
corresponding adaptive cyclic shift selections for a representative
pool of mobile devices;
[0024] FIG. 6 illustrates adaptive cyclic shift selections of
reference signals for four representative mobile devices of FIG. 5
at a different point in time;
[0025] FIG. 7 is a flow chart illustrating adaptive allocation of
cyclic shifts; and
[0026] FIG. 8 is a block diagram illustrating a mobile device that
uses adaptive cyclic shift selection for reference signals.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
[0027] FIG. 1 is a representation of two cells in a cellular
communication network 100 that includes an embodiment of adaptive
cyclic shifting of references signals. In this representation only
two cells 102-103 are illustrated for simplicity, but it should be
understood that the network includes a large matrix of cells and
each cell is generally completely surrounded by neighboring cells.
A representative set of user equipment U1-U2 is currently in cell
102 and is being served by NodeB N1. Cell 103 is a neighbor cell
and NodeB N2 is not serving UE U1-U2. UE U1 and U2 are
representative of a set of user equipment in any given cell since
there will typically be tens or hundreds of UE in each cell. Each
UE communicates with its serving NodeB using an uplink transmission
UL and a downlink transmission DL.
[0028] As mentioned above, in the uplink (UL) of frequency division
multiple access (OFDMA, SC-FDMA, etc) communication systems, it is
beneficial to provide orthogonal reference signals (RS), also known
as pilot signals, to enable accurate channel estimation and channel
quality indicator (CQI) estimation enabling UL channel dependent
scheduling.
[0029] FIGS. 2A and 2B show exemplary sub-frame structures 200A and
200B that includes reference signals, according to an embodiment of
the present invention. Exemplary sub-frame structure 200A is one
possible sub-frame structure used by a UE for UL transmissions in
an OFDMA based system, such as a DFT (Discrete Fourier Transform)
spread OFDMA system or a SC-FDMA system. Exemplary sub-frame
structure 200B is another possible sub-frame structure used for the
same purpose.
[0030] Sub-frame 200A comprises of two short blocks (SBs) 202 and
six long blocks (LBs) 204 and in the exemplary embodiment it is
assumed to have duration of 0.5 milliseconds (msec). All blocks are
preceded by a cyclic prefix transmission 206 to protect the
corresponding data against the channel delay spread and the
respective multi-path propagation. In the exemplary embodiment
200A, data (including control related ones) are assumed to be
transmitted in the LBs while reference signals (RS), also referred
to as pilots, are assumed to be transmitted in the SBs. Combination
of SBs into LBs for the RS transmission may be alternatively
applied. The transmission time interval (TTI) of a UE may extend
over one or several sub-frames. Sub-frame 200B shows another
exemplary embodiment of the 0.5 ms sub-frame structure. In the
sub-frame 200B, the RS could be located in any of the symbols, such
as for example LB#1, which is the first symbol, or LB#4, which is
the middle symbol.
[0031] FIG. 3 presents a block diagram for a transmitter 300 in a
SC-FDMA system. The information bits, after passing through the
coding blocks 302, including an encoder, a CRC attachment and an
interleaver, are provided to the modulating unit of the SC-FDMA
system. After applying a Discrete Fourier Transform (DFT) 308 on
the data, which may also be an ACK/NAK or a CQI related to the
downlink (DL) communication, mapping 310 of the DFT output is
performed on a selected part of the operating bandwidth (BW). This
mapping may be localized, implying that the data sub-carriers
occupy a continuous part of the BW, or distributed, implying that
the data sub-carriers occupy a discontinuous part of the BW.
Subsequently, an Inverse Fast Fourier Transform (IFFT) 312
operation is applied, followed by CP insertion 314, time windowing
316 to produce a signal with the desired spectral characteristics,
a digital-to-analog converter (DAC) 318, and finally the
transmission (Tx) radio frequency (RF) circuitry 320 which includes
a power amplifier and the transmitter antenna. In addition the UE
may be responsive to Node B signaling indicating a transmit time
and/or transmit power adjustment. Similar processing can be applied
for the RS which is not modulated signal (carries no information)
in order to allow the Node B to perform channel related estimation
functions. The RS can be generated from a CAZAC-based sequence 304
and is subsequently cyclically shifted 306 prior to being sent to
the DFT 308 for the same functions as for the data transmission to
occur thereafter. As the DFT of a predetermined RS-sequence (which
can be CAZAC-based sequence) is known in advance, this operation
may be omitted, and sub-carrier mapping 310 of the frequency domain
representation of the RS-sequence can be performed with the cyclic
shift applied after the IFFT 312. This alternate embodiment, where
cyclic shift is applied after the IFFT, is shown in FIG. 5.
[0032] FIGS. 4A and 4B illustrate alternative embodiments of
sub-carrier mapping in the system of FIG. 3. In FIG. 4A, the
mapping 310A is localized as illustrated by the data sub-carriers
occupying a continuous part of the BW with zero padding elsewhere.
In FIG. 4B the mapping 310B is distributed as illustrated by the
data sub-carriers occupying a discontinuous part of the BW with
zeros inserted on intervening sub-carrier slots.
[0033] Mapping unit 310 produces localized and distributed
transmissions in the frequency domain. Control module 311 is
responsive to scheduling commands received on the downlink from the
serving NodeB and configures mapping unit 310 in response to the
received commands. More specifically, the scheduling operation
refers to localized signal transmission in contiguous parts of
bandwidth (BW), referred to as resource blocks (RBs). In the some
embodiments, the RBs assigned to a UE are consecutive, but in
general they may be anywhere in the overall scheduling BW. The
scheduling BW during a given time period is typically only a part
of the total operating BW.
[0034] In order for the Node B to obtain a CQI estimate for the UL
channel of a UE over the scheduling BW, and thereby perform
frequency and/or time domain channel dependent scheduling, the UE
needs to transmit a RS over the scheduling BW or over the entire BW
(distributed RS). On the other hand, in order to minimize losses
from channel estimation, a UE needs to transmit a RS only over the
RBs where the UE transmission is scheduled (localized RS) in order
to avoid unnecessarily dispersing its transmit power over a wider
bandwidth. For this reason, a UE typically transmits a RS, other
than the RS associated with data demodulation (DM RS), over a
relatively wide bandwidth to enable its serving Node B to obtain a
CQI estimate and perform time and/or frequency domain scheduling
for the UE over that bandwidth. This RS effectively provides
channel sounding over its transmission bandwidth and is referred to
as sounding RS (SRS) or CQI RS. As it is typically not the same as
the DM RS, it is transmitted during a different a symbol replacing
data transmission.
[0035] In general, there are two significant limitations in the UL
that make frequency dependent scheduling more difficult than in the
DL. First, the UEs are transmit power limited which makes accurate
CQI estimation challenging, particularly for UEs located near the
geographic boundary of the cell and/or whose signals are received
at the Node B with low signal-to-interference and noise ratio
(SINR). Also, unlike the DL where the CQI estimate may be averaged
over several sub-frames, as a RS is transmitted in all sub-frames,
in the UL this is only possible if a UE transmits a SRS during
consecutive sub-frames, which would result in an unacceptable
increase in UL overhead. The overhead associated with the
transmission SRSs should be less the resulting scheduling gains in
UL throughput. Second, each of multiple UEs devices needs to
transmit a separate SRS for CQI measurements, making the efficient
multiplexing of such reference signals an important issue.
[0036] In the past, the transmission of UE reference signals (RS)
is specified independently of a UE's dynamically estimated delay
spread. The drawback of such solution is that the reference signal
parameters are not tailored to a users dynamically changing delay
spreads, and thus, the resources which are assigned to reference
signals aren't efficiently utilized. As a result, the selected
cyclic shift used by all UEs is typically larger than needed. The
consequence of using a large cyclic shift for all UEs is that there
are fewer RS sequences (different cyclic shifts thereof) available
for allocation to UEs.
[0037] In contrast to prior art, embodiments of the present
invention dynamically allocates transmission parameters of
reference signals, depending on measurements of UEs delay spreads.
In exemplary embodiments, the transmission parameters of reference
signals are cyclic shifts of reference sequences. Reference
sequences can be generated by modifying and modulating CAZAC
sequences, which are sequences with good correlation
properties.
[0038] One construction method for uplink (UL) RS among UEs
belonging in a given pool of multiplexed UEs having RS transmission
over the same bandwidth, is for each UE to transmit a RS formed by
cyclic shift of a Constant-Amplitude-Zero-Auto-Correlation (CAZAC)
sequence, such as a Zadoff-Chu sequence. Multiplexed UEs use
distinct integer multiples of the same baseline cyclic shift. It is
important to note that as the N UEs from the given pool use a
common pool of sub-carriers, the only distinction between their RS
transmissions is the value of the Cyclic Shift. Thus, the resulting
RS signals are said to be orthogonal in the code domain (CDM of the
RS). Different cells may use different base CAZAC sequences.
[0039] To facilitate the understanding of the invention, FIG. 5
illustrates exemplary delay-spread plots and corresponding adaptive
cyclic shift selections for a representative pool of eight mobile
devices. FIG. 5 exemplifies a use of the method for adaptively
specifying the cyclic shift size allocated for orthogonal RS
generation at each UE depending on the channel delay spread and the
timing error which is experienced by each of the simultaneously
multiplexed UEs.
[0040] In typical deployment instances, UEs whose signals
experience large delay spreads will be multiplexed with UEs whose
signals experience low delay spreads. In such scenarios, cyclic
shifts to scheduled UEs for the formation of orthogonal RS can be
allocated adaptively, in accordance to the delay spreads of
scheduled UEs. The cyclic shift value may also be adapted to the
operating environment so that a small cyclic shift value is used in
channels with small signal delay spreads, such as channels
encountered in indoor environments, while a large cyclic shift
value is used otherwise (outdoor environments). Throughout this
document, reference to "a scheduled UE during a given time
instance" means any UE having an UL signal transmission whether it
is just an RS transmission or it includes additional data
signals.
[0041] For a constant cyclic shift value, a multiple of which being
applied to a corresponding multiple of UEs having RS transmission
in the same bandwidth, the number of available cyclic shifts equal
to the mathematical floor of the ratio of the symbol duration
divided by the cyclic shift duration. Therefore, for symbol
duration of 33 .mu.sec and cyclic shift duration of 5 .mu.sec,
there are a total of 6 available cyclic shifts and the RS from 6
UEs can be orthogonally multiplexed through different cyclic
shifts. With adaptive cyclic shift allocation according to the
delay spread experienced by each UE, the multiplexing capacity can
be increased as more cyclic shifts become available. FIG. 5
describes an example of the proposed adaptive cyclic shift
allocation, where 8 orthogonal RS and corresponding UEs are
simultaneously supported even though the worst--case delay spread
is 5 .mu.sec.
[0042] In the embodiment represented by FIG. 5, the time length of
a short block is 33 .mu.sec, referring back to SB1 202 of FIG. 2. A
CAZAC length is selected to require the same amount of time for
transmission. At one given point in time, UE-1 has an estimated
delay spread and timing uncertainty 501 of 2 .mu.sec. Similarly,
UE-2 has an estimated delay spread and timing uncertainty 501 of 3
.mu.sec, UE-7 has an estimated delay spread and timing uncertainty
507 of 5 .mu.sec, and UE-8 has an estimated delay spread and timing
uncertainty 507 of 4 .mu.sec, for example.
[0043] In general, if a total of M UEs are to be multiplexed, the
cyclic shift allocated to the m-th UE is equal to the sum of the
largest (estimated) timing uncertainties and delay spreads of
previous m-1 UEs. Thus, UE-1 is allocated the original sequence
with no cyclic shifts 511. UE-2 is allocated a cyclic shift 512
which equals the timing uncertainty+delay spread of the first UE,
which is 2 .mu.sec, etc. UE-7 is allocated a cyclic shift 517 which
equals to the timing uncertainty+delay spread of the first six UEs,
and UE-8 is allocated a cyclic shift 518 which equals to the timing
uncertainty+delay spread of the first seven UEs. Note that the
original sequence length equals the RS duration. For example, if
the sequence length is 151 and the RS duration is 33.3 .mu.sec,
then the cyclic shift of 1 .mu.sec roughly corresponds to the
cyclic shift of ceil(151/33.3)=5 samples. In general, if the
sequence length is L and the RS duration is T .mu.sec, then a
cyclic shift of T.sub.0 .mu.sec means a cyclic shift of the
sequence by ceil(L * T.sub.0/T) samples, where "ceil" denotes the
mathematical ceiling operation. Other cyclic shifts which are
approximately close to ceil(L * T.sub.0/T) are not precluded [for
example round(L * T.sub.0/T)], where "round" denotes the
mathematical rounding operation of a real number to its closest
integer. Thus, given a mixture of UEs with high and low delay
spreads, the number of supportable UEs with orthogonal RS increases
substantially. Table 1 gives a complete example for the
representative pool of FIG. 5.
TABLE-US-00001 TABLE 1 example allocated shifts at one point in
time Timing uncertainty + delay spread Allocated cyclic shift UE-1
2 .mu.sec 0 .mu.sec UE-2 3 .mu.sec 2 .mu.sec UE-3 4 .mu.sec 5
.mu.sec UE-4 5 .mu.sec 9 .mu.sec UE-5 5 .mu.sec 14 .mu.sec UE-6 5
.mu.sec 19 .mu.sec UE-7 5 .mu.sec 24 .mu.sec UE-8 4 .mu.sec 29
.mu.sec
[0044] In this example, if all of the UEs were allocated the same
cyclic shift based on a worst case timing uncertainty and delay
spread time of 5 .mu.sec, then only six UEs could be included in
the pool instead of eight. The same concept can be extended for the
case the cyclic shift is adapted to the operating environment where
even though a multiple of a base cyclic shift value may be used by
a UE, the base cyclic shift value is selected according to the
operating environment and is not always the largest one
corresponding to the worst operating conditions.
[0045] FIG. 6 illustrates adaptive cyclic shift selections of
reference signals for representative mobile devices of FIG. 5 at a
different point in time. Thus, in this embodiment, the allocated
cyclic shift is updated periodically. As UEs move around in the
cell differences in transmission distance and obstacles will cause
the delay spread of each UE to change. Some delay spread will get
longer and some will get shorter. For example, in FIG. 6, UE-1 now
has a delay spread of 5 .mu.sec which causes an allocated cyclic
shift 602-603 in UE-2 to be 5 .mu.sec. UE-2 now has a delay spread
of 2 .mu.sec which causes an allocated cyclic shift 604-605 in UE-3
to be 7 .mu.sec, etc. Typically, the sum will always be less than
if the worst case amount was assumed for all, therefore more UEs
can be supported in each cell.
[0046] Some downlink (DL) control signaling will be required to
specify the cyclic shift for each UE that is selected based on the
actual UE need (delay spread and timing uncertainty) and not always
set at a maximum value to support a worst case scenario. The number
of bits required to indicate the allocated cyclic shift is very
small assuming some quantization of the smallest possible cyclic
shift corresponding to a lowest delay spread channel and timing
uncertainty and larger cyclic shifts being defined relative to the
lowest one. Moreover, the cyclic shift value allocated to each UE
varies much slower than the sub-frame or TTI duration, assumed to
be in the order of 1 msec, as the delay spread and timing error
remain constant over a much longer period. For example, the delay
estimation for each UE may be updated at about the same rate as its
transmission timing which is in the order of hundreds of
milliseconds or even in the order of seconds.
[0047] FIG. 7 is a flow chart illustrating adaptive allocation of
cyclic shifts. As described above, an estimate 702 is made by the
serving NodeB of the delay spread for each UE in a pool that is
scheduled for transmission at a given point in time. This
estimation is made by measuring a baseline reference signal from
each UE, such as the DM RS or the SRS. The baseline reference
signal may be a different reference signal from those used to
estimate channel estimation (DM RS) and to estimate the CQI
(SRS).
[0048] NodeB then allocates 704 a cyclic shift value for to an m-th
UE based on a sum of cyclic shifts allocated to previous m-1 UEs of
the plurality of UEs scheduled for uplink transmission during the
same transmission time interval or sub-frame, where m=[1, total
number of UEs scheduled for uplink transmission].
[0049] Once all of the UEs in the pool 706 have been allocated a
cyclic shift value, then each UE utilizes the allocated cyclic
shift to form 708 the transmitted RS by applying a corresponding
cyclic shift to the CAZAC sequence, whereby the transmitted RS is
orthogonal to each RS transmitted by all others of the plurality of
UEs scheduled in the same time transmission period or
sub-frame.
[0050] Each UE then transmits 710 the RS in the sub-frame according
to the transmission schedule.
[0051] Periodically 712, each UE transmits another baseline RS and
the estimation 702 and allocation 704 process is repeated.
[0052] FIG. 8 is a block diagram of a UE 1000 with an embodiment
adaptive cyclic shift allocation, as described above. Digital
system 1000 is a representative cell phone that is used by a mobile
user. Digital baseband (DBB) unit 1002 is a digital processing
processor system that includes embedded memory and security
features. In this embodiment, DBB 1002 is an open media access
platform (OMAP.TM.) available from Texas Instruments designed for
multimedia applications. Some of the processors in the OMAP family
contain a dual-core architecture consisting of both a
general-purpose host ARM.TM. (advanced RISC (reduced instruction
set processor) machine) processor and one or more DSP (digital
signal processor). The digital signal processor featured is
commonly one or another variant of the Texas Instruments TMS320
series of DSPs. The ARM architecture is a 32-bit RISC processor
architecture that is widely used in a number of embedded
designs.
[0053] Analog baseband (ABB) unit 1004 performs processing on audio
data received from stereo audio codec (coder/decoder) 1009. Audio
codec 1009 receives an audio stream from FM Radio tuner 1008 and
sends an audio stream to stereo headset 1016 and/or stereo speakers
1018. In other embodiments, there may be other sources of an audio
stream, such a compact disc (CD) player, a solid state memory
module, etc. ABB 1004 receives a voice data stream from handset
microphone 1013a and sends a voice data stream to handset mono
speaker 1013b. ABB 1004 also receives a voice data stream from
microphone 1014a and sends a voice data stream to mono headset
1014b. Usually, ABB and DBB are separate ICs. In most embodiments,
ABB does not embed a programmable processor core, but performs
processing based on configuration of audio paths, filters, gains,
etc being setup by software running on the DBB. In an alternate
embodiment, ABB processing is performed on the same OMAP processor
that performs DBB processing. In another embodiment, a separate DSP
or other type of processor performs ABB processing.
[0054] RF transceiver 1006 includes a receiver for receiving a
stream of coded data frames from a cellular Node B via antenna 1007
and a transmitter for transmitting a stream of coded data frames to
the cellular Node B via antenna 1007. A RS is transmitted to nearby
Node Bs and configuration commands are received from the serving
Node B as described above. Transmission of the RS and scheduled
resource block transmissions are configured as described above. In
this embodiment, a single transceiver supports SC-FDMA operation
but other embodiments may use multiple transceivers for different
transmission standards. Other embodiments may have transceivers for
a later developed transmission standard with appropriate
configuration. RF transceiver 1006 is connected to DBB 1002 which
provides processing of the frames of encoded data being received
and transmitted by cell phone 1000.
[0055] The basic SC-FDMA DSP radio includes DFT, subcarrier
mapping, and IFFT to form a data stream for transmission and DFT,
subcarrier de-mapping and IFFT to recover a data stream from a
received signal. DFT, IFFT and subcarrier mapping/de-mapping may be
performed by instructions stored in memory 1012 and executed by DBB
1002 in response to signals received by transceiver 1006.
[0056] DBB unit 1002 may send or receive data to various devices
connected to USB (universal serial bus) port 1026. DBB 1002 is
connected to SIM (subscriber identity module) card 1010 and stores
and retrieves information used for making calls via the cellular
system. DBB 1002 is also connected to memory 1012 that augments the
onboard memory and is used for various processing needs. DBB 1002
is connected to Bluetooth baseband unit 1030 for wireless
connection to a microphone 1032a and headset 1032b for sending and
receiving voice data.
[0057] DBB 1002 is also connected to display 1020 and sends
information to it for interaction with a user of cell phone 1000
during a call process. Display 1020 may also display pictures
received from the cellular network, from a local camera 1026, or
from other sources such as USB 1026.
[0058] DBB 1002 may also send a video stream to display 1020 that
is received from various sources such as the cellular network via
RF transceiver 1006 or camera 1026. DBB 1002 may also send a video
stream to an external video display unit via encoder 1022 over
composite output terminal 1024. Encoder 1022 provides encoding
according to PAL/SECAM/NTSC video standards.
[0059] As used herein, the terms "applied," "connected," and
"connection" mean electrically connected, including where
additional elements may be in the electrical connection path.
"Associated" means a controlling relationship, such as a memory
resource that is controlled by an associated port. The terms
assert, assertion, de-assert, de-assertion, negate and negation are
used to avoid confusion when dealing with a mixture of active high
and active low signals. Assert and assertion are used to indicate
that a signal is rendered active, or logically true. De-assert,
de-assertion, negate, and negation are used to indicate that a
signal is rendered inactive, or logically false.
[0060] While the invention has been described with reference to
illustrative embodiments, this description is not intended to be
construed in a limiting sense. Various other embodiments of the
invention will be apparent to persons skilled in the art upon
reference to this description. This invention applies to all
scheduled communication systems which perform channel sounding
across multiple resource blocks. This invention applies in uplink
and downlink. Embodiments of this invention are applicable, but not
restricted to, frequency division multiplexed (FDM) reference
signal transmission for simultaneous transmission from multiple
UEs. This includes, but is not restricted to, OFDMA, OFDM, FDMA,
DFT-spread OFDM, DFT-spread OFDMA, single-carrier OFDMA (SC-OFDMA),
and single-carrier OFDM (SC-OFDM) pilot transmission. The
enumerated versions of FDM transmission strategies are not mutually
exclusive, since, for example, single-carrier FDMA (SC-FDMA) may be
realized using the Discrete Fourier Transform (DFT)-spread OFDM
technique. In addition, embodiments of the invention also apply to
general single-carrier systems.
[0061] A Node B is generally a fixed station and may also be called
a base transceiver system (BTS), an access point, or some other
terminology. A UE, also commonly referred to as terminal or mobile
station, may be fixed or mobile and may be a wireless device, a
cellular phone, a personal digital assistant (PDA), a wireless
modem card, and so on.
[0062] In another embodiment, the cyclic shift allocated to each
m-th UE is equal to the sum of a delay spread and a timing
uncertainty of each of the previous m-1 UEs plus the delay spread
of the m-th UE. The term "delay spread" alone also implies
inclusion of timing uncertainties. Multiple antennas of the same UE
can be treated as another separate UE, except that delay-spread
estimation can be common for all antennas of one UE.
[0063] It is therefore contemplated that the appended claims will
cover any such modifications of the embodiments as fall within the
true scope and spirit of the invention.
* * * * *